A fox may grow gray, but never good.
狐狸有可能会变灰，但绝不会变好。
gray 灰色的 grow gray 变灰
A leopard can't change its spots.
豹子不能去掉它身上的斑点。
leopard 豹子，美洲豹 spot 斑点
You cannot make a crab walk straight.
你不能让螃蟹竖着走路。
crab 螃蟹
old habits die hard
积习难改
He's taken up smoking again. You know he had quit for about a year, but Iguess old habits die hard.
他又开始抽烟了，之前他已经戒烟大约一年多了，可能还是积习难改吧。
